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While medicated patients' white matter volume did not differ from that of controls, white matter volume was abnormally small in 49 never medicated patients scanned. These results held up even after the researchers controlled for the fact that the unmedicated children tended to be younger.

Fibers that establish neurons' long-distance connections between brain regions, white matter normally thickens as a child grows older and represents one gauge of the brain's maturation. A layer of insulation called myelin progressively envelops these nerve fibers, making them more efficient, just like insulation on electric wires improves their conductivity.

"Children with ADHD are often described as less mature than their peers and this may relate to delays in white matter maturation," explained Castellanos. "While we do not yet know if medication can accelerate white matter growth, we do know that treating children with medication helps their behavior while they're taking the drugs. There is no evidence that it helps after they stop." Animal studies will be required to determine the impact of medication on brain maturation, he added.

In the current study, ADHD patients' developmental trajectories for nearly all brain regions paralleled growth curves for controls, but on a slightly lower track. "Fundamental developmental processes active during late childhood and adolescence are essentially healthy in ADHD," say the researchers. "Symptoms appear to reflect fixed earlier neurobiological insults or abnormalities." Evidence suggests that ADHD runs in families and may have genetic roots.

As might be expected with hyperactive subjects, the investigators had to discard 50 of 594 total scans due to blurring by motion in the scanner. Volumes of various brain structures and tissue were measured and analyzed by an automated system incorporating more than
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Contact: Jules Asher
NIMHpress@mail.nih.gov
301-443-4536
NIH/National Institute of Mental Health
8-Oct-2002